If you're mailing something and it's destination is in the same state you're mailing it from the USPS says that it can take from two to three days to arrive at it's destination.
When you mail something through the USPS the mail has to go through different sorting facilities which could be further away than the town you're sending the mail to and then it makes it's way to the sorting facility where it will be finally delivered.
When you're mailing something out of state the mail can take up to 7 days to reach it's destination but in the same state the mail should arrive within 3 days assuming it doesn't get lost or any bad weather affects it's delivery.